The present invention relates to a light-emitting diode array and an optical print head provided with such a light-emitting diode array.
Conventionally, light-emitting diodes (LEDs) are widely used as display devices for their advantages of emitting clear light, operating from low voltages, needing simple peripheral circuits, and for other reasons.
Nowadays, optical print heads employing light-emitting diode arrays are much studied for the purpose of finding their practical applications as light sources in optical printers or the like based on electrophotography.
In an optical printer employing as a light source a light-emitting diode array, which emits light by itself, the individual dots of the light-emitting diode array are made to emit light in accordance with an image signal, and the thus emitted light is shone, through a unit-magnification imaging device such as a gradient-index lens, onto a photoconductive drum to form an electrostatic latent image thereon. Then, toner is selectively deposited on the photoconductive drum by a developer unit, and the thus deposited toner is transferred onto plane paper or the like, thereby achieving printing.
A print head employing a light-emitting diode array offers advantages of (1) being free of movable parts and composed of a small number of components, and thus capable of being made compact, and (2) permitting as many array chips as required to be connected together so as to be readily formed to have a desired total length.
In a light-emitting diode array having a light-emitting region density of, for example, 600 dpi (dots per inch), the light-emitting regions are arranged with a pitch of 42 to 43 xcexcm. Thus, considering the width of non-light-emitting regions secured between the light-emitting regions, the width of the light-emitting regions themselves is smaller than that pitch. On the light-emitting regions, electrodes each having a width smaller than 42 to 43 xcexcm are formed with a pitch of 42 to 43 xcexcm. These electrodes formed on the light-emitting regions are connected to wiring patterns that are formed to be so fine as to have a width smaller than 42 to 43 xcexcm each and arranged with a pitch of 42 to 43 xcexcm or smaller. These wiring patterns are in turn connected to wider bonding electrodes (pads). This light-emitting diode array, having electrodes and wiring formed as described above, needs to be electrically connected, by wire bonding or the like, to, for example, a driving device for driving the light-emitting diode array.
Even stitch bonding, which permits bonding with the smallest width at present, requires a bonding width of 40 xcexcm, to which it is inevitable to add a further 20 xcexcm considering the positioning accuracy of bonding equipment. That is, as the width of a bonding electrode, it is necessary to secure about 60 xcexcm at least.
Accordingly, in a high-density light-emitting diode array having a density of 600 dpi or higher, arranging bonding electrodes in a single row makes wire bonding difficult; for this reason, it is customary to arrange them in a zigzag in two rows to secure a greater electrode pitch. Even then, however, it is possible to secure 60 xcexcm at best as the width of a bonding electrode. This has recently been making extremely difficult to electrically connect a light-emitting diode array and a driving device for it together by wire bonding.
Conventional light-emitting diodes and light-emitting diode arrays of this type designed for use in optical printers are disclosed, for example, in Japanese Utility Model Application Published No. H7-36754 and Japanese Patent Applications Laid-Open Nos. H5-347430 and H5-155063.
FIG. 4 is a plan view of the light-emitting diode array for use in an optical printer disclosed in the above-mentioned Japanese Utility Model and Patent Applications, and FIG. 5 is a sectional view taken along line A-Axe2x80x2 shown in FIG. 4.
In these figures, reference numeral 1 represents a compound semiconductor, made of GaP, GaAsP, GaAlAs, GaAs, or the like, that has a plurality of light-emitting regions 11 formed on the top surface thereof by selective diffusion so as to be arranged in a single row or in a zigzag in two rows. Reference numerals 2, 3, and 4 represent insulating films, made of Si3N4, SiO2, AlO3, or the like, that are laid over one another on the top surface of the compound semiconductor 1. These insulating layers 2, 3, and 4 are laid in a plurality of layers to serve as the selectively diffused film of the light-emitting regions, as a protective film for protecting the top surface of the compound semiconductor 1, as a pinhole-prevention film, as a wiring-reinforcement/primary-coating film, and as a light-extraction/brightness-adjustment film. Reference numeral 5 represents an electrode layer, made of Al or the like, that is laid over the insulating films 2 and 3 so as to have ohmic contacts with the light-emitting regions and that serves as electrodes connected to the light-emitting regions, as bonding electrodes, and as wiring patterns that connect those electrodes together. Reference numeral 6 represents a common electrode, made of Au or the like, that is laid on the bottom surface of the compound semiconductor 1.
Here, in the case of a high-density light-emitting diode array having a density of 600 dpi or higher, applying wire bonding directly to the bonding electrodes formed on the top surface of the light-emitting diode array causes the following problems.
To obtain a higher wiring density, it is necessary to reduce the width of the wiring patterns that are connected to the bonding electrodes and reduce the intervals between adjacent wiring patterns. However, if they are reduced extremely, when the wiring patterns are deformed by external force that may be applied thereto during the array cleaning or head assembling process, there is the risk of deformed wiring patterns readily making contact with the adjacent wiring patterns. To prevent this, it is necessary to increase the thickness of the protective films that are formed on the individual wiring patterns to prevent corrosion and the like. For example, in a light-emitting diode array having a density of about 300 dpi, forming 0.2 xcexcm thick inorganic protective films (made of SiO2, SiN, or the like) will suffice; by contrast, in a high-density light-emitting diode array having a density of 600 dpi or higher, such thin films do not provide sufficient protection, and thus a material of the polyimide family is tentatively used as the insulating film 4.
FIGS. 6 to 8 are diagrams illustrating how wire bonding is performed on a light-emitting diode array of which the insulating film 4 is made of, for example, a material of the polyimide family.
As shown in FIG. 6, when the insulating film 4 is made of, for example, a material of the polyimide family, first, a thick-film solution of a material of the polyimide family is spun on the electrode layer 5 to form the insulating film 4 serving as a protective film, and the upper portion of the insulating film 4 is patterned by etching to form openings 4a that permit connection of the wiring patterns. These openings 4a are shaped, for example, as indicated by hatching in FIG. 7, so that comparatively wide connection regions 5a are formed on the electrode layer 5.
Then, as shown in FIG. 8, ball bonding is performed on the connection regions 5a of the electrode layer 5 by using a bonding wire 13 made of gold or the like and dispensed through a capillary 12, and then the thus ball-bonded connection regions 5a are individually connected to the output terminals of the driving device by the bonding wire 13. Alternatively, first, ball bonding is performed on the output terminals of the driving device by using the bonding wire 13, and then the thus ball-bonded output terminals of the driving device are individually connected to the connection regions 5a. In this case, there exist no ball-like portions on the connection regions 5a that are bonded at the second stage of bonding, and therefore, there, the bonding wire 13 needs to be rubbed against the connection regions 5a so as to be deformed and thereby bonded thereto.
However, as shown in FIG. 6, when protection is achieved with a thick film (1 to 2 xcexcm) of a material of the polyimide family, this thick film, lying partially on the connection regions, hampers proper deformation of the bonding wire during bonding, and thus tends to cause insufficient connection. In particular in cases where the bonding wire is bonded to the connection regions at the second stage of bonding, the thick film lying on the connection regions is very likely to act as an obstacle in the height direction and cause imperfect connection. In the worst case, the capillary collides with the thick film, making bonding impossible in the first place. Moreover, insufficient bonding strength increases the risk of wire breakage or the like resulting from the difference in thermal expansion between different metals (usually between Al and Au) or from deterioration with time.
The above description deals with ball bonding, but similar problems are encountered also with stitch bonding that permits high-density connection.
An object of the present invention is to provide a light-emitting diode array, and an optical print head provided with such a light-emitting diode array, that permits wire bonding to be performed securely and thus permits reliable bonding in cases where, for example, resolutions of 600 dpi or higher are required and thus high-density connection by wire bonding is required.
According to a basic feature of the present invention, in a light-emitting diode array having a plurality of light-emitting regions arranged in a row, and having individual electrodes, connected individually by way of wiring patterns to the light-emitting regions, arranged in a plurality of rows, a protective film is laid on the surfaces of the wiring patterns except at least on the individual electrodes.
Here, the protective film may be a thick film formed so as to cover the surfaces of the wiring patterns. Alternatively, the film thickness of the protective film may be equal to or less than the film thickness of the individual electrodes. Alternatively, the protective film may be a thick film made of a material of a polyimide family.
In this way, according to the present invention, a light-emitting diode array, and an optical print head incorporating it, has a protective film laid on the surfaces of wiring patterns except at least on individual electrodes. Accordingly, even in a high-density light-emitting diode array having a density of 600 dpi or higher, it is possible to achieve both satisfactory surface protection of wiring patterns and secure bonding. Thus, it is possible to realize a reliable optical print head.